DESCRIPTION The parent grant will use morphometry, immunohistochemistry, quantitative synaptology and in situ hybridization to investigate the effect of estrogen on the input to GnRH neurons of primates. The present application is an extension of the same aims, with the use of pre-embedding double labeled light and EM immunocytochemistry for GnRH and one of six transmitters that are either inhibitory (GABA, catecholamines, beta-endorphin) or excitatory (galanin, substance P, neuropeptide Y) to GnRH release. In addition, postembedding immunocytochemistry for GABA and glutamate will be used to quantitate their respective inputs to GnRH neurons. The micrographs will be quantified for stained and unstained boutons contacting GnRH positive and negative cells, using a stereological method for quantification in 5 experimental animals per group. The groups will be comprised of primates in endogenous or simulated estrogen surge of mid menstrual cycle, and surgical or naturally menopausal animals, some replaced with estrogen. Similar experiments will be performed on ovariectomized rats, treated with estradiol or control. In general, the treatments and tissue procurements in primates will be performed at the US lab, and rat experiments at both labs. The immunocytochemistry and analysis will be performed at the foreign site.